132 INSECT PESTS 



This is where their importance comes in from the 

 grower's point of view. Take, for instance, the common 

 Potato, Solanum tuberosum. This plant is really a 

 native of South America, which has been introduced 

 into most other countries during the past four or five 

 hundred years, a relatively short period in the scale 

 of evolution. In England the almighty " spud " is able 

 to continue its growth during about seven months of the 

 year, when it has to be protected from the rigour of the 

 seasons. If only that were all the position of the " spud " 

 would be fairly secure, as long as there were human beings 

 to store and protect it, even though it might never become 

 a native plant. But at this point another party has 

 something to say, one Phytophthora infestans, who has 

 strong views, backed up with the power to see them 

 respected. P. infestans comes along and says, " What, 

 Solanum here ! Not if I know it." Away he goes to 

 release his miUions of invisible agents, and good-bye to 

 your potato crop by the time Phytophthora and the Hke 

 have done their fell work. 



The potato, however, is only one of the plants which 

 have to suffer from the attacks of fungi, and to meet 

 these we must apply the same principles to the manage- 

 ment of our crops as we have seen to be necessary in 

 the case of insects. In a word let us find out where to 

 break the chain of development in the plant parasites 

 which trouble us. Again we must study hfe histories 

 in order to arrive at the weak spot at which to apply 

 our check. 



The life history of a fungus, although they show many 

 variations, is somewhat as follows : 



Existence commences in the form of a Spore, a simple 

 cellular body, practically invisible, and capable of being 

 blown about by the merest puff of air until it alights upon 

 the host plant, where it takes root and grows. Sometimes 

 there are two or even three host plants, all different, yet 

 the fungi on each are merely preliminary stages in the 



